One aspect of the invention relates to twin screw mixer/extruders, but more particularly, another aspect of the invention relates to a torque splitting transmission.
The relatively small distance required between the axes of the screw shafts of a twin screw extruder has historically limited the diameters of final drive pinion gears of a torque splitting transmission for driving the closely spaced twin screws. The pinion gear size is limited as a function of the center distance between the shafts. In one extreme, the diameter of the driven pinion gears cannot be greater than the distance between two co-rotating shafts. In another extreme where the drive gear pinions are axially offset from the other, one of the pinion gears cannot have a radius that is greater than the difference between the distance between the shafts and the radius of one shaft.
Unfortunately, the torque capacity of a gear varies with the cube of its diameter and the small diameter of the pinion gear(s) is a limiting factor of the torque that can be delivered to the twin screws whose torque capacity is typically greater than the torque capacity of the pinion gear(s). In operation, twin screws generate an axial force that is carried as a thrust load into the torque splitting transmission. There are several prior art solutions to enhance the torque limitations associated with a small diameter pinion gear and bearing loads induced by the pinion gear and twin screws.
The techniques for enhancing the torque capacity of 35 torque splitting transmissions with closely spaced output shafts involve avoiding overloading relatively small teeth of the pinion gear. For example, the output torque capability of a transmission may be enhanced by using helical gears instead of spur gears for the pinion gear to increase the number of teeth in contact with each other such as disclosed in U.S. Pat. No. 4,261,225. Another way of avoiding overloading the pinion gear teeth and enhancing torque output capability using spur gears, is to use planetary gearing to engage two teeth of spur type pinion gears as is disclosed in U.S. Pat. No. 3,824,875. Another gear arrangement involving spur teeth pinion gears is shown in U.S. Pat. No. 4,315,440 where two teeth of the pinion gear are in contact with other spur gears for the purpose of halving the tooth loading to the small diameter pinion gear. A way of insuring that the tooth loading is proportionately shared between two teeth of a pinion gear is shown in the elaborate arrangement of gearing disclosed in U.S. Pat. No. 4,399,719 where a radially movable drive pinion gear is used to split torque into two components.
The prior art solutions show that the output torque of a transmission may be at best doubled by engaging two teeth of a small diameter pinion gear whose diameter is limited by the distance between the axes of the twin screw extruders. The torque output capability of such gearing is less than the torque handling capability of the twin screws and thus, is a limiting factor in the output capacity of the twin screw extruder.
Another problem associated with a pinion gear is that it generates radial forces which must be carried by bearings which are limited in size because of the small distance between the axes of the twin screws.
In other words, the small diameter of the pinion gear limits the amount of power that can be transmitted to this gear and the power delivered by the gear is below that capable of being received by the screws.